The International System of Units (ISO) in 1960 is metric and was one of many attempts this century to standardize metric bolt sizes internationally.
The ISO Standards
The ISO standards are widely used, and bolt diameters range from 2 to 20mm in three varieties of metric thread pitch: the standard, fine and extra fine varieties. The JIS (Japanese Industrial Standards) sizes range from 0.4 to 1.5 for the same set. A further standard, almost identical to JIS is the German DIN (Deutsches Institut fÅ±r Normung).
For someone on a tight budget, the 10, 13 and 15mm are considered a good buy, as so many cars use them. However, the 17 and 18mm do pop up on others. Many people would buy a set because the bolts you need are always the one you don’t get, and if you buy piecemeal, you pay more overall.
Others feel that it is more valid to use the origin of the car as an indicator. In other words, a Japanese made car needs the 8, 10, 12, 14, 17, 19, 21mm and no others. European cars need 10, 13, 15, 19 and 21mm.